Due to the need to reduce costs and rationalise operations in the woodworking and furniture industry – and also in the related engineering sector – the optimisation of peripheral components such as those used for loading and unloading machines, is becoming increasingly important.

In many cases, the loading of the machines is the bottleneck which prevents utilisation of the full capacity and productivity of the machine tools (such as saws, planning machines, milling machines) which have been optimised for top performance over the last few years.

In the global marketplace, woodworking companies in the highly industrialised countries can compete successfully against companies in low-wage countries only if they employ innovative concepts and automate all of their production processes. This has been shown clearly in the past few years.

Since wood is a naturally growing raw material, its handling places particularly high demands on vacuum technology. The same applies to products made from wood.

Typical examples of the problems involved are the porosity of the wood, which can vary widely, the surface quality of the sawn wood (from smooth to very rough), the possible presence of knots, cracks and twisted boards, the varying dimensions, the undefined position of the parts to be gripped, the presence of sharp edges, and other contributory aspects such as uneven stacking and contamination with resins and dust. (see figure 3 and 4)

This article deals with the lifting of complete layers of boards, strips and planks of wood with the aid of a complete vacuum gripper system (VacuBox) in applications where warping of the parts (as the result of the shorter drying process) and contamination with dust make the working conditions more difficult.
In spite of these problems, the users demand the maximum system availability (operation in two or even three shifts per day), fast handling and operational reliability.

Conventionally, the mechanical loading of a woodworking machine is done by so-called tilt unstacking. The pallet on which the boards are stacked is lifted and tilted. When the top layer of boards rises above an edge, gravity causes the boards slide off the stack onto a conveyor belt which then feeds them singly into the machine.

The main drawback of this relatively cheap method is that the boards often jam when they slide off the stack.
Rough-sawn and warped boards tend to tilt up during this process, which means that they land in an unorderly heap on the conveyor belt and must be straightened manually by additional persons. This not only costs time and money, but also incurs the risk of injuries to these persons.

Further critical aspects of the mechanical process are that it easily damages the boards and takes up a lot of space in the workshop. (see figure 5 and 6)

In contrast, handling with the VacuBox maintains the original orderly stacking of the boards ("intelligent handling by lifting in layers"). This ensures a high level of process reliability and system availability which, in turn, makes additional personnel unnecessary, and transport of the boards with little damage.

Basically, workpieces with varying dimensions and/or undefined positions can be gripped with a device which has multiple vacuum grippers spaced on a grid, where the spacing between the individual grippers is selected to meet the demands of the application. (see figure 7)

However, this method cannot be used successfully for parts with rough surfaces, since the sealing lips of the rubber suction pads do not adapt themselves well to such surfaces.

In contrast, a large-area vacuum gripper with a foam-rubber sealing mat is capable of handling rough and structured surfaces, since the mat provides a much better seal.

Standardisation and cost aspects also play a role, since solutions with multiple grippers generally have to be designed and manufactured individually.

The large-area vacuum gripper system VacuBox is a modular system which uses the following components: (see figure 8)

1. Vacuum reservoir with valves

Main task: To permit fast gripping and release of the layer of boards to be lifted in order to achieve short cycle times.

Secondary aspects: Flow-optimisation, taking costs and weight into account.

The use of a separate vacuum reservoir with valves in the VacuBox ensures an extremely high volume flow during the critical gripping process. This high flow rate means that even warped boards are picked up and gripped securely for safe transport.
The boards are released by venting the vacuum to the atmosphere. If extremely short cycle times are needed, the release operation can be accelerated by an optional blow-off function, which uses compressed air.

2. Valves

Main task: Permit gripping and lifting of workpieces with varying dimensions and undefined positions (including boards with knot-holes and other openings).

Secondary aspects:

• Minimisation of the necessary evacuation capacity and thus the energy consumption of the vacuum generator
• Achievement of the highest possible system vacuum for maximum holding forces
• Insensitivity to dirt and foreign objects
• Even when handling badly warped boards, the valves should not close too quickly, since this would cause the vacuum chambers to be deactivated and the workpiece would not be gripped

The above requirements are fulfilled by the use of leak-free, self-acting check valves with a patented technology, designed to be self-cleaning and thus unaffected by dirt and dust. Each valve closes automatically if its related vacuum chamber is not in contact with the workpiece, thus permitting the overall system vacuum to be maintained.

3. Blower

Main task: To provide the necessary volume flow and vacuum

Secondary aspects:

• High volume flow during gripping in order to pull the (possibly warped) boards against the VacuBox.
• High maximum vacuum in order to provide the necessary holding force for the handling of heavy boards.
• Robustness

The vacuum blower used here ("Schmalz Blower") is a directly driven blower whose volume flow/vacuum characteristic and robustness have been optimised to meet the above requirements.
In order to ensure a long operating lifetime, the vacuum blower is fitted with a pressure-limiting valve (which opens if the blower is inadvertently left running at the maximum vacuum) and a filter (to prevent dust from being drawn into it).

4. Sealing mat

Main task: To provide a seal with the demanding workpiece surface (rough, dusty, sharp-edged, angled) in order to apply the vacuum to the workpiece.

Secondary aspects:

• Must be soft and flexible
• At the same time, it must be hard-wearing for a long operating lifetime
• Its leakage rate must be as small as possible so that the smallest possible vacuum generator can be used

In order to meet these requirements, special, hard-wearing sealing mats made of foam rubber have been developed. These have a standard arrangement of holes to permit gripping and lifting of boards with all common formats used in the woodworking industry.

Due to the special importance of the sealing mat for the entire VacuBox system (particularly with respect to the operating costs), this deserves to be treated in more detail.

As already mentioned, the leakage through the sealing mat has a considerable effect on the necessary size of the vacuum generator. A higher leakage rate means that more air has to be evacuated, which means that a larger vacuum generator is needed and this, in turn means a higher investment and higher operating costs.
The leakage rate of the sealing mat should remain at a constant low value for the longest possible time during the operating lifetime of the mat and with the mat subjected to the normal deformation stresses.

Cost situation

A small example shows how the mat quality affects the investment and operating costs: (see figure 9)

• Purchase price for a low-cost sealing mat:    50€
• Purchase price for a high-quality sealing mat:   150€

Due to the higher porosity of the low-cost sealing mat (particularly after some initial wear of the mat), a larger blower is needed to compensate for the higher leakage.

• Blower power needed for vacuum gripper with low-cost mat:  7.5kW
• Blower power needed for VacuBox with high-quality mat:   4.0kW

Energy costs for 3 months of operation (70 working days, 10 hours/day, 0.1€/kWh)

• Energy costs with low-cost sealing mat:  70 x 10 x 7.5 x 0.1 = 525€
• Energy-costs with high-quality sealing mat:  70 x 10 x 4.0 x 0.1 = 280€

In spite of the considerable difference between the purchase prices for the two mats, the high-quality sealing mat pays for itself in this example in just over one month.

The above comparison ignores the fact that a larger blower costs more to purchase (price difference between 4 kW and 7.5kW blower: about 2,500€) and maintain than a smaller blower. Furthermore, the high-quality sealing mat has a considerably longer operating lifetime, which means that the costs for purchasing replacement mats and installing them (with the related machine downtimes) are also far higher when low-cost mats are used.

Sealing mats of competition

Some competitors attempt to minimise this considerable drawback by coating the cell chambers of the sealing mat manually with a layer of silicone. Admittedly, this (initially) reduces the leakage through the sealing mat, but also incurs the risk of reduced flexibility of the mat (which means that it does not adapt itself so well to the workpiece surface), the undefined flaking of the silicone layer (which can result in a sudden loss of the sealing function) and, of course, the far higher costs for the additional step of coating with silicone. (see figure 10 and 11)

The use of modern sealing mats thus permits the use of smaller vacuum generators, with a corresponding positive effect on the costs for the overall system.

Modular large-area vacuum gripper system FX

As the result of the successful further developments in sealing-mat technology, it is in some cases already possible to use large-area vacuum grippers without an additional vacuum reservoir, even for difficult applications.

For an example of this, let us look at the comprehensive, modular large-area vacuum gripper system Type FX.
The design of the FX gripper is basically similar to that of the VacuBox, but it does not have a vacuum reservoir. This has a considerable effect on the weight and price of the system and also permits the use of cheaper vacuum generators driven by compressed air (ejectors) instead of blowers. (see figure 12 and 13)

In general, the FX gripper tends to be used in applications where the dimensions, weight, degree of warping and surface roughness of the boards to be handled do not present such great problems as those in applications where the VacuBox is typically used.

However, the decision as to which solution should be used for a given application must be made by an expert, particularly as the further developments in sealing mats and valve technology are currently very dynamic.

Finally, I would like to refer to something which, at the first glance, does not seem logical.

Typical requirement:

The typical application for the VacuBox is the handling of boards, strips and planks of varying dimensions.
The board width generally lies between 50 and 200 mm and the layer width between 1,000 and 2,000 mm (in Europe, it is frequently 1,200 mm). The length of the boards lies between 3,000 and 6,000mm, whereby layers with boards of different lengths may occur but are very rare. In most cases, all of the boards n a layer have the same length. (see figure 14)

Solution 1: One central VacuBox

If one central VacuBox is used to handle boards of different lengths, it is necessary to provide a mechanical traversing facility in order to position the box centrally for each length, since long overhangs at one end could have a negative effect on the function of the box and on the lifetime of the sealing mat.
This makes it necessary to purchase, install and maintain a drive motor, gearbox, guides, controller, etc. (see figure 15 and 16)

Solution 2: Several smaller VacuBoxes

If several smaller VacuBoxes are used, this can be mounted in fixed positions, saving investment and operating costs, since no mechanical traversing device is necessary. (see figure 17 and 18)

Further advantages of the "multi-box concept":

Advantage

• Boxes can be deactivated individually when, for example, only short boards are to be handled for some time. 
• Smaller boxes have a smaller area of contact with the workpiece. 
• The vacuum reservoir can be "loaded" to a higher vacuum value.

Benefit

• Reduced energy and operating costs, flexible adaptation to various operating conditions.
• In general, they can adapt themselves better to warped or curved boards.
• Better gripping behaviour and shorter cycle times.
  
  

In addition to the above aspects, the purchase price for a large VacuBox with a large vacuum generator is the same as, or only slight less than, the price for several small VacuBoxes with separate, small vacuum generators.

It should also be mentioned here that the large central VacuBoxes are, for reasons of manufacturing, flexibility and modularity, normally constructed as a "beam version" (where beams carrying the valves are flange-mounted on the vacuum reservoir). For smaller VacuBoxes, in contrast, the version with integrated functions (valves, valve chamber and reservoir in a single housing) is generally better.

To sum up, it can be said that the principle "more VacuBoxes = better and cheaper" applies from both the technical and the economical viewpoints, whereby the complete handling system, including its mechanical components must be considered.

However, depending on the specific requirements, there are also cases where the use of a single, central VacuBox is the better solution.

Once again, only an expert can decide which solution is best for a given application. Furthermore, due to the previously mentioned wide variations in the porosity and surface roughness of the workpieces (wood is not a standardised product), it is also advisable to carry out some tests with original workpieces in order to achieve reliable operation and the best possible price-performance ratio.

The intelligent automation of workpiece handling offers a considerable potential for increasing the productivity and economic use of woodworking machines by optimising the stacking and unstacking operations.

Vacuum gripping and handling systems are becoming increasingly popular, particularly in large-scale production, since they permit full utilisation of the cost benefits which result from the higher process reliability of such automated systems, up to and including unmanned operation.

The VacuBox is a complete vacuum gripper system which is optimised to meet the complex demands of the woodworking industry for the fully automatic handling of layers of boards and planks, is designed for continuous operation and provides a rational solution for the related applications.

A particularly important feature of these devices is the sealing mat, whose further development in the recent past has led to considerable technological advances and to reduced costs for purchase and operation.

If all relevant aspects with respect to investment volume, floor area, energy costs, personnel costs, consumption and maintenance costs, cycle times and the damage-free handling of the workpieces are taken into account, a solution with vacuum is better than a purely mechanical solution, particularly in the areas of operating reliability and the reduction of the machines downtimes.